In some communication systems, a transmitting device sends data to a receiving device by sending light signals representing the data through an interconnecting optical fiber. Data to be sent over an optical fiber may first be represented by digital electrical signals which are converted to corresponding modulated light signals. The modulated light signals travel through the optical fiber to one or more receiving devices. A receiving device detects modulated light signals passing through the optical fiber, converts the modulated light signals into digital electrical signals, and sends the digital electrical signals to other parts of the communication system.
A transceiver is a combination of a transmitting device and a receiving device. The transmitting device, also referred to as a transmitter, converts input electrical signals comprising data to be sent through the network to a signal that is compatible with the physical medium of the network. For example, the transmitter in an optical transceiver converts input electrical signals into modulated light signals for output to an optical fiber. The receiving device, also referred to as the receiver, detects signals carried by the physical medium of the network and adapts the signals for output to other devices connected to the transceiver. In the example of the optical transceiver, the receiver detects light signals from an optical fiber and converts the light signals into electrical signals representing data received from the network.
In order to facilitate interoperability over a network comprising equipment from different suppliers, transceivers may be required to send and receive signals having signal parameters in accord with a telecommunications standard. One such standard is the Synchronous Optical Network (SONET) standard, published by the Exchange Carriers Standards Association for the American National Standards Institute. The SONET standard includes requirements for signals to be exchanged over a network having optical fibers as the physical medium of the network, more commonly known as a fiber optic network.
In SONET and some other networks, data is transmitted serially without an accompanying clock signal. A clock signal is a means of establishing a common time reference for actions in different parts of a system. As signals corresponding to data pass into and through the transmitter portion of a first optical transceiver, over an optical fiber, and into and through the receiver portion of a second optical transceiver located some distance from the first transceiver, amplitude, timing, and phase errors may be introduced into the signals. These errors may introduce uncertainty into the measurement of timing parameters used to recover data from the signal received by the second transceiver. For example, the errors may cause unwanted variations in time intervals and durations used to convert signal pulses to data bits. Such unwanted variations in signal parameters related to timing are referred to as jitter. Jitter may cause output data from the second optical transceiver to differ from input data to the first optical transceiver, which may result in a network transmission error. To avoid this undesirable result, the SONET standard includes specifications related to a maximum value of jitter that may be introduced into signals transmitted over the network by devices connected to the network. Standards other than SONET may also include specifications related to jitter limits.
Optical transceivers are among the devices that operate within jitter limits expressed in the SONET standard. The parts of an optical transceiver contributing the most jitter to transceiver output signals are the multiplexer (MUX) and demultiplexer (DMUX) circuits. A MUX merges n parallel data input lines, each input line having a data rate of m bits per second, into a serial data output line have a data rate of n×m bits per second. A MUX is sometimes referred to as a serializer. A DMUX is complementary to a MUX, forming n parallel data output lines, each parallel output line having a data rate of m bits per second, from a serial data input line having a data rate of n×m bits per second. A DMUX is sometimes referred to as a deserializer. In an optical transceiver, a mux is generally part of the transmitter and a DMUX is generally part of the receiver.
Many high speed optical transceivers include components fabricated using semiconductor processes such as silicon CMOS and silicon-germanium (SiGe) BiCMOS technologies. For an optical transceiver adapted for data rates up to 10 gigabits per second (Gbps), also referred to as a 10 G transceiver, jitter from transceiver components fabricated from these processes generally does not exceed SONET limits. However, circuits fabricated from these processes tend to have increased jitter at data rates above 10 Gbps. For example, it is not unexpected that about half the number of transceiver components made from these processes will be unusable in 40 Gbps transceivers due to jitter measurements in excess of SONET jitter limits.
In addition to jitter originating within semiconductor components, conducted and radiated noise from sources external to the transceiver may interact with transceiver circuits and contribute to jitter measured at the transceiver's outputs. Some optical transceivers use components having special packaging to shield internal circuits from noise and reduce jitter in transceiver outputs. FIG. 1 illustrates an example of a packaged transceiver component, in this case a receiver. FIG. 1 is an example of a receiver adapted for operation at data rates up to 40 Gbps, also known as a 40 G receiver, for use in a 40 Gbps transceiver. FIG. 1 is also representative of packaging for other 40 Gbps transceiver components known in the art, for example a transmitter, modulator driver, optical modulator, or optical receiver. As shown in FIG. 1, the prior art packaged 40 G receiver 100 is enclosed in a machined metal enclosure 102. An enclosure 102 having one or more shielded connectors 108 is known in the art as a connectorized package. A butterfly package is another metal package similar to FIG. 1. Generally, laser diodes have butterfly packages. Because of its relatively large size and weight, the metal enclosure 102 of FIG. 1 includes mounting flanges 104 for attaching the 40 G receiver 100 to a circuit card assembly with mechanical fasteners such as bolts or rivets. A mechanical fastener may also be used to establish a ground connection between a mounting flange 104 and the circuit card assembly.
One or more shielded connectors 108, which may be of a type referred to as V connectors, GPPO connectors, or similar connectors for high-frequency signals, are used to connect a shielded cable assembly carrying high-frequency signals to the packaged 40 G receiver 100. A plurality of electrical contacts 110 are used for low-speed signal, direct current (DC) power, and ground connections between components inside the packaged 40 G receiver 100 and an external circuit card assembly. Electrical contacts 110 are electrically isolated from the metal housing 102 by insulators made of ceramic or some other electrical insulator. A fiber optic cable 112, a short piece of which is shown in FIG. 1, connects from a communication network to the receiver 100 through a fiber optic connector and strain relief 106. The fiber optic cable, which comprises an optical fiber for carrying optical signals, is part of the communication network and is not part of the transceiver comprising the receiver 100.
Connectorized and butterfly packages are expensive to manufacture and install, especially in comparison to surface mount packages. Furthermore, shielded connectors can exacerbate jitter problems and may cause other problems such as signal attenuation, reflections, and reduced system reliability. Shielded cable assemblies for electrical signal connections between components having connectorized packages are bulky, require manual assembly into the transceiver, and may be about as expensive as the components they connect. For a connectorized package having a pair of shielded connectors 108 for connection of a differential signal, as in FIG. 1, two shielded cables are required for signal connections to the receiver 100, raising space requirements and installed costs even more.
FIG. 3 is a block diagram of an example of a 40 Gbps optical transceiver known in the art. The 40 Gbps optical transceiver 300, also known as a 40 G transceiver 300, comprises a transmitter 342 and a receiver 344. The transmitter 342 of 40 G transceiver 300 comprises a MUX 306 for merging data on sixteen parallel transceiver input data lines 302, aligning the serialized data with a transceiver input clock signal 304, and producing a serial 40 Gbps output data signal carried on a data cable 308 attached to an output from the connectorized MUX 306 package. The serial data output signal from the MUX 306 on the 40 Gbps data cable 308 includes jitter introduced by the MUX, connectors, cables, and external noise sources. In some cases, jitter from the MUX 306, which may be made from SiGe BiCMOS technologies for 40 Gbps operation, may comprise about 80% of the total jitter measured at an optical output of the prior-art transceiver 300.
An example of a shielded cable assembly for high frequency electrical signal connections to shielded connectors is shown in FIG. 4. The shielded cable assembly 600 of FIG. 4 comprises two metal connectors 602 attached to a cable 604. One shielded cable assembly 600 is used for each electrical signal coupled between shielded connectors on connectorized packages. FIG. 4 is representative of the 40 Gbps data cable 308, the modulator cable 340, and a second 40 Gbps data cable 332 used in the receiver 344 (see below).
The optical modulator 312 of FIG. 3 modulates light output from a laser diode 314 to create an optical signal that is coupled into a 40 Gbps optical output 316 and then into an optical fiber 318. In some transceivers, the optical modulator 312 and the laser diode 314 are replaced with an electro-absorptive modulator laser (EML), a device which outputs an intensity-modulated light signal in response to a modulated input voltage. The 40 Gbps optical signal at an optical output 316 and then entering an optical fiber 318 comprises data to be transmitted and jitter accumulated from the MUX 306, the modulator driver 310, the optical modulator 312, plus jitter accumulated from the various connectors and cable assemblies along a signal path from an input to an output of the prior-art transceiver 300.
The receiver portion of 40 G transceiver 300 in FIG. 3 comprises optical receiver 324 and a device 334 which combines a 1:16 DMUX and a Clock and Data Recovery (CDR) function in one package. A CDR forms a clock signal from timing information extracted from a data signal in a data line. A 40 Gbps input signal comprising data to be received is carried through an optical fiber 320 coupled into a 40 Gbps optical input 322 on the transceiver 300. The optical receiver 324 is in a connectorized package comprising a photodetector 326, a transimpedance amplifier 328, and a limiting amplifier 330. The optical receiver 324 receives light signals from the 40 Gbps optical input 322 and outputs a digital electrical signal comprising serial data having a data rate of 40 Gbps on a 40 Gbps data cable 332 connected to an input of 1:16 DMUX CDR 334, also in a connectorized package. The 1:16 DMUX CDR 334 converts the 40 Gbps serial data input to sixteen parallel data outputs 336, each output carrying digital data at a date rate of 2.5 Gbps and jitter added to the data signal by the optical receiver 324, the DMUX CDR 334, and connectors and shielded cable assemblies linking the connectorized packages for the DMUX CDR and optical receiver. For a DMUX CDR 334, a substantial amount of jitter may be added to the sixteen 2.5 Gbps outputs 336 from the 40 G transceiver 300. For transceiver components known in the art, such as the receiver 344 and the transmitter 342 of FIG. 3, jitter is cumulative along a signal path from an input to an output.
Components for optical transceivers for use in SONET networks are routinely tested to reject units that do not meet jitter specifications. For example, MUX and DMUX, devices are screened for acceptable levels of jitter before the devices are installed in prior-art transceivers. As previously noted, production yields for some components used in prior-art 40 G transceivers are about fifty per cent, a figure determined mostly by MUX and DMUX devices having measured jitter in excess of a selected test limit. Yield figures may be improved incrementally by using connectorized packages and shielded cable assemblies, but these solutions have other problems as previously described.
What is needed is a circuit topology for 40 G transceiver components that is relatively insensitive to jitter from semiconductor devices, enabling transceivers to be built from components having jitter in excess of SONET jitter limits. Additionally, what is needed is a topology for transceiver components that have SONET-compliant outputs but do not have connectorized packages, shielded connectors, or shielded cable assemblies for interconnections between components.